With some apologies to our neighbours to the south, Canada's plant hardiness maps have always been more thorough than the US Department of Agriculture maps. While the USDA maps relied only on average minimum temperatures, the Canadian version incorporated both minimum and maximum temperatures, frost free periods, summer and winter precipitation, and wind factors. The new map, released recently, also incorporates elevation into its calculations. Sunset magazine has created a similar map for the US, but with 45 different zones, instead of the usual 8 or 10.

Besides the confusion over zones and maps, there is always the problem that hardiness zones were designed for trees and woody shrubs, not perennials. Woody plants are always somewhat exposed to the weather, while perennials, which die back over the winter, can often survive quite cold temperatures if their roots are insulated by a sufficient snow cover.

So if all these variables leave you questioning the suitability of a specific plant for your area, Canada's new Plant Hardiness Site should answer those questions. While still a work in progress, the site aims to collect data from gardeners across the continent and compile a growing range map for each species. Simply search for the plant you are considering, by either the common or scientific name, and it will return to you a map of its range. The maps show where it has been observed growing (green dots), its core range (light green) and its general range (brown).

Or are you wondering what you can grow? Click on Plant Lists by Area of Interest for a list of plants that should grow in your area, or plants that have been submitted by other gardeners as growing in your area. You can even get projections of what might grow there in the future (up to 2100!) according to their climate change model. As more gardeners contribute to this project, the maps will improve in accuracy, and will probably become a very valuable resource. So contribute to the project today!